An example in which the first device is a wireless base station and the second device is a private branch exchange is used for the following explanation.
A mobile station that is wirelessly connected to the wireless base station converts a sound analog signal, which is obtained from the voice of a user of the mobile station, to a sound digital signal. The sound digital signal remains unchanged when being transmitted from the mobile station to a telephone of an intended person via the wireless base station, the private branch exchange, and the like. The telephone of the intended person reproduces the sound analog signal from the sound digital signal, making it possible for the intended person to listen to the sound.
If noise gets mixed into the sound digital signal on a transmission line of the sound digital signal, the sound analog signal, too, is affected by the noise. In particular, if noise gets mixed into a heavily weighted portion of the sound digital signal, an excessive sound is generated at the telephone of the intended person, making the intended person feel uncomfortable.
One of the reasons why noise gets mixed into the sound digital signal is the disconnection of a line between the wireless base station and the private branch exchange.
Conventionally, as for the detection of the disconnection of a line to which a base station interface card of the private branch exchange and the wireless base station are connected, the following have been used to detect the disconnection and recovery, thereby suppressing the excessive sound: the monitoring of line voltage by hardware, and the monitoring of communication state by software of the private branch exchange. In the case of a conventional hardware-based suppressing means, if the line disconnection and the recovery take place for a short period of time, the excessive sound could be transmitted to the exchange at the time of recovery. In the case of a software-based suppressing means, the detection cycle is long depending on the specifications of the exchange; therefore, until the line disconnection is detected by software, the excessive sound could be transmitted to the exchange.
What is disclosed in PTL 1 is a line disconnection detection method, according to which: in an in-band signal transmission device that uses a transmission line for transmitting and receiving a sound signal and uses a first frequency signal including an in-band single frequency signal or a plurality of frequency signals to transmit and receive a signaling signal for an exchange or telephone, a signal generation means that a transmission unit includes is used to generate a second frequency signal that is different from the sound signal and the first frequency signal; during a talk situation, the second frequency signal is transmitted from the signal generation means to the other station at a time when a non-talk situation is detected by a level detection means in such a way that the sound signal of the other station and the second frequency signal are detected by the level detection means that a receiving unit includes; and it is determined that the line is disconnected at a time when the second frequency signal is not detected by the level detection means. Moreover, the following is disclosed in PTL 1: According to the line disconnection detection method, a no-signal detection means that the transmission unit includes is used to detect a non-signal state of a transmission sound and the first frequency signal; the disconnection of the line at the time of signaling or non-talking is detected by the no-signal state detection of the no-signal detection means; and the disconnection of the line during talking is detected by a no-signal state of the second frequency signal by the level detection means.